1. Field of the Invention
The present invention relates to an active material, an electrode containing the same, a lithium-ion secondary battery equipped with the electrode, and a method of manufacturing the active material.
2. Related Background Art
Laminar compounds such as LiCoO2 and LiNi1/3Mn1/3CO1/3O2 and spinel compounds such as LiMn2O4 have conventionally been used as positive electrode materials (positive electrode active materials) for lithium-ion secondary batteries. Attention has recently been focused on compounds of olivine-type structures such as LiFePO4. Positive electrode materials having olivine structures have been known to exhibit high thermal stability at high temperatures and achieve high safety. However, lithium-ion secondary batteries using LiFePO4 are disadvantageous in that their charge/discharge voltage is low, i.e., about 3.5 V, thus exhibiting low energy density. Therefore, LiCoPO4, LiNiPO4, and the like have been proposed as phosphate-based positive electrode active materials which can achieve high charge/discharge voltage. Nevertheless, the lithium-ion secondary batteries using these positive electrode materials have not yet attained sufficient capacity. Among the phosphate-based positive electrode materials, LiVOPO4 has been known as a compound which can achieve 4-V class charge/discharge voltage. However, even lithium-ion secondary batteries using LiVOPO4 have failed to attain sufficient reversible capacity, rate characteristic, and cycle characteristic. The above-mentioned positive electrode materials are disclosed in the following literatures, for example. In the following, the lithium-ion secondary battery will be referred to as “battery” as the case may be.
Japanese Patent Application Laid-Open No. 2003-68304
Japanese Patent Application Laid-Open No. 2004-303527
J. Solid State Chem., 95, 352 (1991)
N. Dupre et al., Solid State Ionics, 140, pp. 209-221 (2001)
N. Dupre et al., J. Power Sources, 97-98, pp. 532-534 (2001)
J. Baker et al., J. Electrochem. Soc., 151, A796 (2004)
Crystals expressed by the structural formula of LiVOPO4 have been known to reversibly insert and desorb lithium ions. The structures expressed by the structural formula of LiVOPO4 have been said to have different crystal structures such as those of α and β types, the α type being a structure thermodynamically more stable than the β type. Japanese Patent Application Laid-Open No. 2004-303527 discloses the making of LiVOPO4 having a β-type crystal structure (orthorhombic crystal) and LiVOPO4 having an α-type crystal structure (triclinic crystal) by a solid-phase method employing V2O5 and their use as electrode active materials for nonaqueous electrolyte secondary batteries. Japanese Patent Application Laid-Open No. 2004-303527 also reports that the discharge capacity of a battery using LiVOPO4 having the α-type crystal structure (triclinic crystal) is lower than that of LiVOPO4 having the β-type crystal structure (orthorhombic crystal).
J. Baker et al., J. Electrochem. Soc., 151, A796 (2004) discloses a method (carbothermal reduction (CTR) method) which heats VOPO4 and Li2CO3 in the presence of carbon, so that carbon reduces VOPO4, thereby producing LiVOPO4 having the β-type crystal structure. J. Solid State Chem., 95, 352 (1991) discloses a method of making LiVOPO4 having the β-type crystal structure by using tetravalent vanadium.